Titanium wire and wire rope



United States Patent 3,511,622 TITANIUM WIRE AND WIRE ROPE Milton A. Nation, 905 Moraga Drive, Los Angeles, Calif. 90045 No Drawing. Filed Oct. 12, 1965, Ser. No. 495,313 Int. Cl. B21f 9/00 US. Cl. 29-193 8 Claims ABSTRACT OF THE DISCLOSURE A high performance wire rope in which there are a plurality of individual wires consisting predominantly of titanium, twisted into axially symmetric wire rope, and thereafter prestressed as necessary to a level approaching the elastic limit of the wires before subjecting the rope to ultimate use, to provide increased torsional stability and minimum kinking. Titanium cable according to this invention provides unique corrosion resistance to result in level performance, and inertial properties to result in high performance at low fatigue rates.

The combination of mechanical properties of titanium wire together with the range of process variables effective during wire and cable production results in inertial characteristics uniquely suited to high performance systems required for aerospace and hydrospace.

This invention relates generally to wires and cables and particularly relates to titanium wire and wire rope of improved mechanical properties.

Titanium metal and most titanium alloys have some special mechanical properties which make up for the relatively high price and the difficulties of processing titanium or titanium alloy wires. Among these properties are long fatigue life and high corrosion resistance particularly to salt water or salt water spray, due to the presence of an oxide film on the titanium. In addition, titanium has a high tensile strength and has the highest strength-to-weight ratio of all commonly used metals, such as ferrous metals and alloys.

For this reason titanium wire rope or cable has been found suitable for use in many commercial and naval multipart reeving systems and in high performance military systems.

One of the problems with any type of conventional wire rope is the presence of non-recoverable or nonelastic stretch, sometimes called irreversible stretch. This is often referred to as constructional stretch because it occurs after the wire rope has been installed and as evident, it is not effectively and permanently removed prior to installation. This is to be distinguished from the elastic stretch which is, of course, recoverable and can be readily calculated in view of the loads and the inertial forces imposed on the wire system on the basis of the modulus of elasticity of the material and the wire rope.

It may be convenient at this point to define various technical terms which will be used hereinafter. Thus, the yield point of the metal may be defined as the stress at which plastic deformation takes place under constant or reduced load. The elastic limit of a metal is the limiting value of the deforming force beyond which the metal does not return to its original shape.

It might be noted that yield strength is sometimes determined by commercial practise, that is, by use of 2% off-set technique. It is not always found to be precisely accurate. Further, the elastic limit is found to be very close to yield strength. This is similarly true for titanium alloys, but no precise technique has been developed for a determination of elastic limit values.

Prestressing of ferrous alloys must be limited to a maximum of about 60% of the ultimate or tensile strength ice of the material. If a cable, for example, is prestretched to a higher percentage of the ultimate strength or breaking load, the fatigue increases and the life expectancy correspondingly decreases. A number of ferrous alloys such as carbon and low alloy steels, so called ultra highstrength steels, and austenistic stainless steels have a large gap between the elastic limit or yield point and their ultimate strength. It might be noted that the three types of steel alloys referred to hereinabove are those steels which are used for making wire and wire ropes. The average size of the gap between the elastic limit and the ultimate strength of these steel alloys is between 30% and 40%.

As a result of this only partial prestressing of wire rope of ferrous alloys there remain various undesirable characteristics. Obviously, there remain further constructional or non-recoverable stretch which gradually works out during use and, of course, causes rigging problems or adjustment problems of the rope. Furthermore, life remains in the cable so that the cable has a tendency to coil unpredictably. This, of course, makes handling very difficult. Finally, the cable is more likely to kink, particularly when it has not been preformed. However, this problem inherently cannot be solved with any wire rope of ferrous alloys because it is not possible to stretch the individual wires or the wire rope beyond 60% of the minimum breaking load due to the existence of the large gap above referred to.

It is accordingly an object of the present invention to provide a cable or wire rope consisting of monofilaments or individual wires, each consisting predominantly of titanium which has substantially no constructional stretch.

Another object of this invention is to provide titanium wire rope or titanium wire which has better reproducible mechanical performance, substantially no constructional stretch in the case of a rope, and which has a high fatigue life.

A further object of the invention is to provide a titanium wire rope as well as wire and a process of making such a wire or wire rope with inproved mechanical properties and where a balance between flexibility and rigidity as well as torsional stability may be obtained.

In accordance with the present invention, use is made of the low gap between the ultimate strength or breaking load and the yield point or elastic limit of titanium metal. This permits straightening of a wire or prestressing of a wire rope of monofilaments or individual wires consisting primarily of titanium, very close to the ultimate strength or breaking load of the titanium or titanium alloy. This processing of the wire in conjunction with the unique and superior mechanical properties of titanium permits the use of lighter and smaller diameter cable. This, in turn, reduces cost as well as aerodynamic or hydrodynamic drag.

Furthermore, titanium has a very narrow gap between the yield strength and the ultimate strength. The existence of this narrow gap permits to operate cable systems very close to the cable breaking load. By way of explanation some figures may be helpful showing the gap between the yield strength and the ultimate strength of titanium metal and various ferrous alloys. Thus, the minimum, the average and the maximum gap of all ferrous alloys is respectively 13%, 25% and 45%. The same figures for ferrous alloys suitable for wires such as carbon and low alloy steels, ultra high strength steels and austenitic steels are 25 35% and 45%. In contrast, the minimum, average and maximum gap for titanium is respectively 2%, 8% and 13%. For a particular titanium alloy consisting of 13% vanadium, 11% chromium, 3% aluminum, the remainder being titanium, the minimum, average and maximum gap is respectively 2%, 4% and 6%. Thus, the

extremely low gap for titanium and its alloys compared to ferrous'alloys will be evident.

Strangely enough, the resistance to fatigue of titanium metal when stretched very close to the elastic limit remains high. Thus, a rope having a plurality of individual wires, each consisting primarily of titanium has been stressed in accordance with the present invention before using it between about 80% and about 95% of its elastic limit or even higher. Thus, a process in accordance with the present invention consists of the step of prestressing the rope before its ultimate use to between about 80% and about 95% or higher of the elastic limit.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description.

It has been found that prestressed wire rope of titanium permits highly uniform and reproducible dynamic performance at a low fatigue rate. Individual titanium wires have been prestressed to immediately below the elastic limit. It has been found that the tensile strength of individual wires is only reduced by less than 5%. This reduction is quite minor and will vary between different titanium alloys.

The following technique may therefore be used in prestressing titanium wire rope. The elastic limit and yields strength in relation to ultimate strength is established for each structural wire alloy of titanium. This may be done by use of published hand book data for other structural forms which do not include wire or by tensile test for strength and elongation.

A level of prestressing is established as a percentage of the breaking load which is equal to the ultimate tensile strength multiplied by the rope eiiiciency for the type of wire rope construction under prestressing. Depending upon the accuracy required this value can be obtained by calculation from the tensile strength and breaking loads of the structural wires used. Alternatively, it may be established by a breaking load test.

The percentage used is dependent upon the prestressed condition desired. Normally the elastic limit of an individual wire should not be exceeded except to improve the elastic condition generally of all wires as this may be safely done, nor should it be less than two-thirds of the breaking strength if constructional stretch is to be effectively removed. It is recognized that certain of these wires receive slightly higher stresses than others and, therefore, the percentage level may be correspondingly lower than the elastic limit. A safe fixed level is established for each type of construction.

Prestressing tension and timing are accurately controlled so as to achieve the accomodation and set desired throughout the rope structure between adjacent wires so as to eliminate constructional stretch. When the highest degree of homogeneous prestressing is required, it is necessary to first remove the coiling cast from structural wire. This removes torsional instability so as to obtain the desired maximum effect. This processing prevents any tendency of the wire to coil after structural wire is fabricatel into wire rope since the helical pattern is completely stabilized.

In order to prestress wire rope, equipment is needed for stressing rope at about 300 lbs., and the range should extend to about 200,000 lbs. If the cable exceeds a diameter of 1%", greater capacity equipment may eventually be needed. It will be realized that it is necessary, of course, to control accurately the prestressing limit which is very near the elastic limit.

This may, conveniently and precisely, be done by means of a known cross-head engine which can be used in accordance with the present invention for continuously and smoothly prestressing long lengths of cable. The cable may be reeved about gangs of sheaves with one cross-head movable under hydraulic control while the cable is roved to the sheaves and cross-head as a system. Such cross-head engines are known and are used, for example, in carrier landings of aircraft on aircraft carriers. For example, cable spools having a length of 15,000 of cable may be rove on to the cross-head mechanism used in accordance with the invention for stretching the cable in" say 1000 increments until the entire length ha s been prestressedto the desired level of stretch.'

Prestressing in accordance with the invention may be done up to near the elastic limit such as about of the tensile strength and may go as high as to obtain wire rope with very high stability and efficiency.

It will be realized that a cross-head engine of the type above referred to and installed in place, for example, on an aircraft carrier, can be used to check a wire rope accurately for further stretch after the purchase cable in service has been prestressed. A cross-head engine may also be used for large cables for any particular application in accordance with the invention. If it is found that the cable has become overstressed in use, the cable would be hazardous for further service due to the severity of the cable dynamics. Thus, the cross-head engine may be used as a check on the future life expectancy of the cable and its performance on a safe strength basis. Thus, the amount of non-recoverable or constructional stretch may be used as an indicator of the remaining service life of the cable.

This indication of service life may be combined with the long established criterion for ferrous wire of broken wires in the cable which is an uncertain indication that the ferrous cable is about to break. Due to the uniform properties of high quality structural titanium or titanium alloy and the resulting uniform performance of titanium cable, both criteria could prove to be most significant and precise with the new cable of the present invention.

Properly processed wire of this elasticity does not break so easily because of flaws or stress raisers, but rather because of fatigue, and occasionally of wear, which may be reduced by the use of a suitable lubricant.

Prestressing of the wire rope not only removes constructional stretch, but in so doing, it changes the performance characteristics of wire rope in the following particulars. 'It increases torsional stability to improve performance in tension modes and during handling. The reason is that individual and adjacent wires become better accommodated and only elastic stretch remains in these modes. It reduces the kinking tendency which is promoted by torsional instability as found when constructional stretch from. preformed wires remains. It adds a stiffness quality which is useful in certain applications. However, the stiffness may be worked out if this quality is undesirable. It adds a quality for reproducing performance in dynamic systems which have a cycling characteristics because constructional stretch is removed.

The net result achieved in prestressing then is a function of the degree of prestressing, the diameter or size of the wire rope and its type of construction. Therefore, the prestressing technique is flexibly and selectively useful in titanium wire rope processing above 50% of the minimum breaking load. The most effective percentage level of prestressing should be determined in each application based upon the performance and service parameters but will generally be between about 80% and about 95%.

It should be particularly noted that impact loads occur in some dynamic systems so that under such circumstances a maximum capability for energy absorption is a preferred performance characteristic. In that eventv constructional stretch, as found in preformed wire rope, should be completely removed so that constructional play, and in turn, internal abrasion is minimized.

The prestressing technique is far more effective in titanium wire rope than in steel wirerope because of the wider range of prestressing which may be-used. This tech-. nique, when combined withwire and wire 'rope size, and type of construction thus provides useful design parameters which set the performance and handling characteristics of the wire rope. This combination of parameters adds flexibility to the performance and service obtained for multipart reeving systems and new high performance systems.

The use of straightened wire combined with prestressing to the elastic limit of the wires not only removes all constructional stretch for reducing rigging problems, but in dynamic systems, stretch is then confined to elastic stretch which can be calculated precisely for the loads being handled. Moreover, degradation in service is confined to slow wear since fatigue life is high and there is high resistance to corrosion. The result is repeated dynamic performance without material change for a long period of service. Some constructional stretch may return unless straightened wire is used.

Such performance represents major improvement, for example, in aircraft control cables. In contrast, for a steel cable all constructional stretch cannot be removed under elfective specifications because of fatigue. To further illustrate design flexibility swaging of all wires, using an effective process, can be carried out to obtain a stiffness quality sometimes needed in dynamic systems such as aircraft control cables in lieu of flexibility.

It should be noted that the monofilaments or wires of which the rope is made up may consist of various suitable titanium alloys containing predominantly titanium. Among these alloys may be mentioned a so-called all-beta alloy wire, consisting of titanium with 13% vanadium, 11% chromium and 3% aluminum. Another titanium alloy is referred to as alpha-beta alloy and consists of titanium with 6% aluminum and 4% vanadium.

In accordance with the present invention it may be preferred to remove the coiling cast, or prevent its formation, either during or after the wire drawing process is completed. As a result, at the conclusion of processing, the structural Wire is straight for use in monofilament structures. Normal coiling cast in drawn wire materially increases the kinking hazard and also the torsional instability found diflicult in handling during spooling or level winding.

Conventional technique for removing coiling cast is by means of a series of rollers which can be conveniently placed in line with the drawing equipment during production to reduce processing cost. Prestressing or prestretching structural wire in accordance with the invention will remove only a small part of this cast, unless the wire is warm drawn which spoils high tensile properties. Therefore, rolling the wire through a series of rollers, a limited workhardening process, does not reduce tensile strength for most conditions. However, roller straightening and prestretching may be used in combination. On the other hand, it has been found that perstressing wire reduces tensile strengths up to 5% when not acting as a strengthening mechanism. Thus, for certain monofilament end uses structural wire may have to be preformed in this manner to make it straight.

Again the prestretching may be carried out in the manner previously described, that is, to between about 80% and about 95% of the elastic limit of the wire or even higher. Such a prestretched wire has improved torsional stability, repeatability and superior dynamic performance. It will also reduce the tendency of the wire to kink. The result of prestretching the wire is to obtain repeatable dynamic performance. Such wire has very uniform mechanical properties and may also be of high quality, which, of course, means that the resulting wire has a uniform and predictable performance.

There has thus been disclosed a titanium wire, and wire rope of improved mechanical properties and performance characteristics, and a process of making the rope as well as the wire. Since the wire rope has been prestressed, substantially all irreversible or constructional stretch has been removed. As a result, the kinking tendency of axially symmetric wire rope is practically eliminated by the prestressing of the invention. In addition, periodic rigging of cable systems or adjustment of aircraft cables is greatly reduced or substantially eliminated. The performance of the wire rope is reproducible under the same vigorous dynamic and environmental conditions over a longer period than with conventional wire rope.

The invention and its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement of the parts of the invention without departing from the spirit and scope thereof or sacrificing its material advantages, the arrangement hereinbefore described being merely by way of example and we do not wish to be restricted to the specific form described or uses mentioned except as defined in the accompanying claims, wherein various portions have been separated for clarity of reading and not for emphasis.

I claim:

1. A rope having a plurality of ductile, flexible wires each consisting of titanium base alloy, said rope being stressed between about and of the elastic limit of said wires.

2. A rope having a plurality of ductile, flexible wires each consisting of titanium base alloy, said rope being formed to minimize constructional stretch and stressed to between about 80% and 95 of the breaking load to stabilize torsion and length in dynamic states for protracted periods.

3. A rope having a plurality of ductile, flexible wires each consisting of titanium base alloy, said rope being formed to minimize constructional play and stressed to less than the elastic limit to stabilize torsion and length, and to prevent kinking in dynamic states for protracted periods.

4. A rope having a plurality of ductile, flexible wires each consisting of titanium base alloy, said wires being straight and without coiling cast, and said rope being designed to minimize constructional play, and being stressed to a predtermined percentage of the breaking load whereby (a) level and reproducible performance at high fatigue life is obtained, and (b) a lighter and smaller diameter cable is permitted in aerodynamic and hydrodynamic environments to suit dynamic modes and to achieve new superior performance.

5. A ductile, flexible wire consisting of titanium base alloy, being stressed between about 80 and about 95% of its elastic limit.

6. A ductile, flexible wire, consisting of titanium base alloy, being in a rolled and stressed condition, free of decided coiling cast, having high tensile strength and high elasticity, and adapted to retain said high strength and high elasticity under sustained repetitive dynamic cycling.

7. The wire of claim 5 wherein the titanium base alloy is all-beta category material of the composition Ti-13V-11Cr-3Al, thereby, to sustain repetitive dynamic cycles at a high performance level on a protracted basis.

8. The wire of claim 5 wherein the titanium base alloy is alpha-beta category material of the composition Ti-6Al-4V, thereby, to sustain repetitive dynamic cycles at a high performance level on a protracted basis.

References Cited UNITED STATES PATENTS 2,214,709 9/1940 Peskin 57-166 X 2,675,309 4/1954 Vordahl 75-175.5 2,804,409 8/1957 Kessler et a1 75-1755 X 2,906,654 9/1959 Abkowitz 75175.5 X 3,147,115 9/1964 Vordahl 75175.5 X 3,394,036 7/1968 Parris 148-115 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R. 57166 

